Patterned Substrate For Light Emitting Diode and Light Emitting Diode Employing the Same

ABSTRACT

Disclosed herein are a patterned substrate for a light emitting diode and a light emitting diode employing the patterned substrate. The substrate has top and bottom surfaces. Protrusion patterns are arranged on the top surface of the substrate. Furthermore, recessed regions surround the protrusion patterns. The recessed regions have irregular bottoms. Thus, the protrusion patterns and the recessed regions can prevent light emitted from a light emitting diode from being lost due to the total reflection to thereby improve light extraction efficiency.

CROSS REFERENCE RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/KR2006/005061, filed on Nov. 28, 2006, and claims priority from andthe benefit of Korean Patent Application No. 10-2006-0012433, filed onFeb. 9, 2006, which are both hereby incorporated by reference for allpurposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a substrate for a light emitting diodeand a light emitting diode employing the same. More specifically, thepresent invention relates to a patterned substrate for a light emittingdiode and a light emitting diode employing the same.

2. Discussion of the Background

GaN-based light emitting diodes (LEDs) have been developed and appliedfor about 10 years. The GaN-based LEDs have significantly changed LEDtechnologies and currently employed in a variety of applicationsincluding full color LED displays, LED traffic lights, white LEDs andthe like.

Recently, high-efficiency white LEDs have been expected to replacecommon fluorescent lamps. Particularly, the efficiency of white LED isapproaching that of general fluorescent lamps. However, there is roomfor further improvement in the efficiency of LEDs, and thus, furtherefficiency improvement is continuously required.

Two major approaches have been attempted to improve the efficiency ofLEDs. The first approach is to enhance the internal quantum efficiencydetermined by the crystal quality and the epitaxial layer structure, andthe second approach is to increase the light extraction efficiency.

Since the internal quantum efficiency currently reaches 70˜80%, there islittle room for further improvement of the internal quantum efficiency.However, there is room for improvement in the light extractionefficiency. For the improvement in light extraction efficiency, it isimportant to eliminate internal loss by employing a heat radiationstructure and roughened surfaces.

Meanwhile, roughened surfaces are employed to prevent total reflectiondue to a difference between refractive indexes of a GaN-based LED andthe surrounding thereof, e.g. a substrate and the atmosphere. Since aGaN-based semiconductor material has a high refractive index of about2.4, a critical angle is relatively large. Light directed toward asurface of an LED at an angle less than a critical angle is totallyreflected and then returned again to the interior of the LED. Such lightmay be again reflected and then radiated to the outside, but a portionof the light is absorbed into the LED or electrodes and then lost in theform of heat. The roughened surfaces prevent light incident on the LEDsurface from being returned to the interior of the LED due to the totalreflection, and thus, allow the light to be radiated to the outside.

Meanwhile, a technology for employing a patterned sapphire substrate toimprove light extraction efficiency has been proposed in “InGaN-basednear-ultraviolet and blue-light-emitting diodes with high externalquantum efficiency using a patterned sapphire substrate and a meshelectrode,” Japanese Journal of Applied Physics, Vol. 41, 2002, pp.L1431-L143, Dec. 15, 2002.

According to the technical paper, a sapphire substrate is etched to formconvex hexagons thereon such that light loss due to total reflection oflight between an LED and a substrate can be reduced to thereby improvelight extraction efficiency.

However, the light extraction efficiency of LEDs has not yet reached asatisfactory level, and thus, continuous efforts to improve the lightextraction efficiency are required.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a patterned substratefor a light emitting diode capable of further improving light extractionefficiency.

Another object of the present invention is to provide a light emittingdiode employing a patterned substrate capable of further improving lightextraction efficiency.

To achieve the objects of the present invention, there is provided apatterned substrate for a light emitting diode and a light emittingdiode employing the patterned substrate. The substrate for a lightemitting diode according to an aspect of the present invention has topand bottom surfaces. Protrusion patterns are arranged on the top surfaceof the substrate. Furthermore, recessed regions surround the protrusionpatterns. The recessed regions have irregular bottoms. Therefore, sincelight emitted from a light emitting diode can be irregularly scatteredby the protrusion patterns and the recessed regions, light loss due tothe total reflection can be reduced and light extraction efficiency canalso be improved.

The substrate may be a sapphire substrate. Since sapphire has arefractive index of about 1.7, the sapphire substrate has a criticalangle larger than that of a SiC or GaN substrate and light loss due tothe total reflection is also relatively large. Therefore, the light losscan be greatly reduced by the protrusion patterns and the recessedregions.

In addition, the bottom surface may be a flat surface. That is, thebottom surface is not patterned. Therefore, the total reflection occurswell on the bottom surface and the totally reflected light is returnedtoward the light emitting diode and then emitted to the top of thediode. Therefore, light emitted outward through the bottom surface ofthe substrate is reduced and thus light extraction efficiency can befurther improved.

The recessed regions surrounding each of the protrusion patterns may besymmetrical with respect to the relevant protrusion pattern. Further,the protrusion patterns may have a variety of sections, e.g. a triangle,a rectangle, a pentagon, a hexagon and the like. A top surface of theprotrusion pattern may be convex upward.

Sidewalls of the recessed regions may be formed into inclined surfaces.The inclined surfaces are formed in such a manner that the width of theprotrusion pattern is increased downward. Therefore, light incident onthe inclined surfaces is generally incident to the substrate.

A light emitting diode employing a patterned substrate according toanother aspect of the present invention comprises a substrate with topand bottom surfaces. Protrusion patterns are arranged on the top surfaceof the substrate. Further, recessed regions surround the protrusionpatterns. The recessed regions have irregular bottoms. Furthermore, afirst conductive type semiconductor layer is formed on the top surfaceof the substrate. Further, a second conductive type semiconductor layeris formed on a region of the first conductive type semiconductor layer,and an active layer is interposed between the first and secondconductive type semiconductor layers. Therefore, there is provided alight emitting diode whose light extraction efficiency is improved bymeans of the protrusion patterns and the recessed regions.

A buffer layer may also be interposed between the substrate and thefirst conductive type semiconductor layer. The buffer layer can reducelattice mismatch between the first conductive type semiconductor layerand the substrate.

According to a further aspect of the present invention, there isprovided a method of manufacturing a patterned substrate for a lightemitting diode. The manufacturing method of the present inventioncomprises the step of preparing a substrate with flat top and bottomsurfaces. A first mask pattern is formed on the top surface of thesubstrate to define first recessed regions. Subsequently, the substrateis partially etched using the first mask pattern as an etching mask toform the first recessed regions and the remaining first mask pattern isthen removed. A second mask pattern is also formed on the top surface ofthe substrate to define second recessed regions. The second recessedregions are formed to partially overlap the first recessed regions.Subsequently, the substrate is partially etched using the second maskpattern as an etching mask to form the second recessed regions.Accordingly, a patterned substrate for a light emitting diode withprotrusion patterns and recessed regions are manufactured.

The first mask pattern may include lines parallel to each other.Further, the second mask pattern may include lines which intersect theparallel lines of the first mask pattern. Therefore, the protrusionpatterns each of which has a rectangular section are formed, and thus,the recessed regions surrounding the protrusion patterns are formed.

Sidewalls of the first and second recessed regions may be formed intoinclined surfaces. The inclined surfaces may be formed by causing thefirst and second mask patterns to be inclined.

A buffer layer, a first conductive type semiconductor layer, an activelayer and a second conductive type semiconductor layer may be formed onthe substrate. Subsequently, the second conductive type semiconductorlayer and the active layer are etched to expose a region of the firstconductive type semiconductor layer. Accordingly, there is provided alight emitting diode formed on the patterned substrate.

According to embodiments of the present invention, there is provided apatterned substrate for a light emitting diode wherein protrusionpatterns and irregular recessed regions can be formed on a top surfaceof the patterned substrate to improve light extraction efficiency. Inaddition, there is provided a light emitting diode wherein the substrateis employed to further improve light extraction efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a patterned substrate for a light emittingdiode according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along line A-A′ in FIG. 1.

FIG. 3 is a sectional view of a light emitting diode employing thepatterned substrate according to an embodiment of the present invention.

FIGS. 4 to 6 are plan and sectional views illustrating a method ofmanufacturing a patterned substrate for a light emitting diode accordingto an embodiment of the present invention.

FIG. 7 is a sectional view of a light emitting diode employing apatterned substrate according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. Thefollowing embodiments are provided only for illustrative purpose tofully convey the spirit of the present invention to those skilled in theart. Thus, the present invention is not limited to the embodiments whichwill be described below, but may be implemented in other forms. In thedrawings, the width, length, thickness, etc. of components may beexaggerated for the sake of convenience. Throughout the descriptions,like reference numerals designate like elements.

FIG. 1 is a plan view illustrating a patterned substrate 10 for a lightemitting diode (LED) according to an embodiment of the presentinvention, and FIG. 2 is a sectional view taken along line A-A′ in FIG.1.

Referring to FIGS. 1 and 2, the substrate 10 has bottom and topsurfaces. Protrusion patterns 17 are arranged on the top surface of thesubstrate 10. The protrusion patterns 17 are arranged in atwo-dimensional way and may be arranged in a matrix form as shown inFIG. 1. However, the present invention is not limited thereto but thepatterns may be arranged in various shapes including a honeycomb shape.Although it is illustrated that the protrusion patterns 17 are formedinto a rectangular shape, they are not limited thereto but may havevarious shapes including a triangle, a pentagon and a hexagon other thana rectangle. Further, although a top surface of each of the protrusionpatterns 17 may be a flat surface, it is not limited thereto but may bea convex surface.

Each of the protrusion patterns 17 is surrounded by first and secondrecessed regions R1 and R2. The first and second recessed regions R1 andR2 surrounding each of the protrusion patterns 17 can be symmetricalwith respect to the relevant protrusion pattern 17, respectively.Further, the first and second recessed regions R1 and R2 may have thesame shape as each other.

Meanwhile, the first and second recessed regions R1 and R2 partiallyoverlap each other. A region 11 where the recessed regions overlap eachother has the bottom deeper than the other regions. Therefore, the firstand second recessed regions R1 and R2 have the bottoms with irregulardepths.

That is, as shown in FIG. 2, a depth D3 from the top surface of theprotrusion pattern 17 to the regions 11 (concave portions) where thefirst and second recessed regions overlap each other is greater thandepths D1 and D2 from the top surface of the protrusion pattern 17 toconvex portions 13 and 15 in the first and second recessed regions,respectively. Further, the depths D1 and D2 may be identical to eachother, or they may be different from each other as shown in the figure.

Sidewalls of each recessed region R1 or R2 define inclined surfaces.Each of the inclined surfaces may have a constant slope, but it is notlimited thereto, and the curvature of the slope may vary.

According to this embodiment, there is provided a substrate having theprotrusion patterns 17 and the recessed regions with irregular bottoms.Therefore, light incident on a top surface of the substrate 10irregularly changes its traveling path, and thus, light loss due to thetotal reflection can be reduced. Particularly, according to thisembodiment, since irregular bottom surfaces are formed in the recessedregions R1 and R2, the top surface of the substrate 10 has surfaces withvarious heights and inclined surfaces. Therefore, since the substrate ofthe present invention can change a path of light more irregularly ascompared with a conventional patterned sapphire substrate, light lossdue to the total reflection can be further reduced.

FIG. 3 is a sectional view illustrating a light emitting diode employingthe patterned substrate 10 according to the embodiment of the presentinvention.

Referring to FIG. 3, a first conductive type semiconductor layer 25, anactive layer 27 and a second conductive type semiconductor layer 29 arepositioned on the substrate 10 as described referring to FIGS. 1 and 2.A region of the first conductive type semiconductor layer 25 is exposedto the outside. Furthermore, a buffer layer 21 may be interposed betweenthe first conductive type semiconductor layer 25 and the substrate 10.The buffer layer 21 is used to reduce lattice mismatch between the firstconductive type semiconductor layer 25 and the substrate 10. Inaddition, an undoped semiconductor layer 23 may be further interposed onthe buffer layer 21.

The first and second conductive type semiconductor layers are of N-typeand P-type, or P-type and N-type, respectively. Each of the first andsecond conductive type semiconductor layers may be formed of an (Al, In,Ga)N compound semiconductor. N-type and P-type semiconductor layers maybe doped with Si and Mg, respectively. The active layer 27 may alsoformed of an (Al, In, Ga)N compound semiconductor, and formed into asingle or multiple quantum well structure.

Meanwhile, an electrode pad 33 may be formed on the exposed region ofthe first conducive semiconductor layer 25, and an electrode 31 and anelectrode pad 35 may also be formed on the second conductive typesemiconductor layer 29. The electrode 31 may be formed into a meshshape. However, the electrode is not limited thereto and may be formedinto a conductive layer of ITO or Ni/Au.

According to this embodiment, light emitted from the active layer 27 istransmitted through the electrode 31 and then radiated to the outside.Further, light emitted toward the substrate 10 is not totally reflectedregularly on the top surface of the substrate 10 but irregularly changesits traveling path such that it is emitted to a side of the substrate orreturned toward the active layer 27. The light returned toward theactive layer 27 changes its traveling path and is thus easily emitted tothe outside. Therefore, the light loss in electrode or semiconductorlayers due to repetition of the total reflection can be reduced tothereby improve the light extraction efficiency.

FIGS. 4 to 6 are plan and sectional views illustrating a method ofmanufacturing a patterned substrate for a light emitting diode accordingto an embodiment of the present invention. Here, FIGS. 4 a and 6 a areplan views and FIGS. 4 b and 6 b are sectional views taken along lineA-A′ in FIGS. 4 a and 6 a, respectively.

Referring to FIGS. 4 a and 4 b, a first mask pattern 51 defining firstrecessed regions R1 is formed on a substrate 10 having top and bottomsurfaces. The substrate 10 may be a sapphire, SiC or GaN substrate.

The first mask pattern 51 may be a photoresist pattern. However, it isnot limited thereto but may be a pattern of a dielectric layer made ofsilicon oxide (SiO2). The photoresist pattern may be obtained by forminga photoresist layer and then performing photolithography and developmentprocesses on the photoresist layer. At this time, the developedphotoresist layer may be subjected to a reflowing process such that itstop surface has a width greater than that of its bottom surface. Thereflowing process may be performed by heating the substrate 10 at atemperature of 100 to 150°C.

Meanwhile, the mask pattern using a dielectric layer may be obtained byforming a dielectric layer on the substrate 10 and then patterning itusing photolithography and etching processes. In this case, sidewalls ofthe mask pattern may be formed into inclined surfaces.

As shown in the figures, the first mask pattern 51 can include parallellines. Therefore, the parallel recessed regions R1 are defined.

Subsequently, the first recessed regions R1 are formed by partiallyetching the substrate 10 using the first mask pattern 51 as an etchingmask. At this time, an etched depth of the substrate 10 may be the depthD1 which is roughly equal to the convex portions 13 of FIG. 2.

Referring to FIG. 5, after the etching process has been completed, theremaining first mask pattern 51 is removed. The first mask pattern 51can be removed using an ashing or etching process.

Referring to FIGS. 6 a and 6 b, a second mask pattern 55 defining secondrecessed regions R2 is formed on the substrate 10 with the firstrecessed regions R1 formed therein. At this time, the second recessedregions R2 partially overlap the first recessed regions R1. That is, thesecond recessed regions intersect the first recessed regions R1. In acase where the first mask pattern 51 includes parallel lines, the secondmask pattern 55 may include lines intersecting the parallel lines of thefirst mask pattern 51. The second mask pattern 55 may be formedperpendicular to the first mask pattern 51, but the present invention isnot limited thereto. Furthermore, the second mask pattern 55 may be aphotoresist pattern or a pattern of a dielectric layer as describedabove.

Subsequently, the substrate 10 is partially etched again using thesecond mask pattern 55 as an etching mask. As a result, the secondrecessed regions R2 are formed. At this time, the etched depth of thesubstrate 10 may be the depth D2 which is roughly equal to the convexportions 15 of FIG. 2. Since the second recessed regions R2 partiallyoverlap the first recessed regions R1, the overlapped regions arerepeatedly etched and are thus etched in the depth D3 corresponding tothe sum of the depths D1 and D2. As a result, the substrate 10illustrated in FIGS. 1 and 2 has been completed.

According to this embodiment, the substrate 10 with the protrusionpatterns 17 and the recessed regions R1 and R2 formed on the top surfacethereof is manufactured. The recessed regions R1 and R2 have irregularbottoms. Further, the first and second recessed regions R1 and R2 areetched to the different depths D1 and D2, and thus, the convex portions13 and 15 in the recessed regions can have different heights.

Although it has been illustrated in this embodiment that the substrateetching processes are performed twice, the present invention is notlimited thereto. That is, the substrate etching processes may beperformed more times. Further, the first and/or second mask patterns maybe formed into various shapes in addition to the line patterns.Therefore, recessed regions having irregularities with various depthscan be formed, and thus, a variety of protrusion patterns can be formed.

A light emitting diode can be fabricated by forming semiconductor layerson the substrate 10 according to this embodiment. Hereinafter, a methodof fabricating a light emitting diode by employing the substrate 10 willbe described.

Referring back to FIG. 3, a first conductive type semiconductor layer25, an active layer 27 and a second conductive type semiconductor layer29 are formed on the substrate 10 having the protrusion patterns 17 andthe recessed regions R1 and R2. Before the first conductive typesemiconductor layer 25 is formed, a buffer layer 21 may be formed.Furthermore, an undoped semiconductor layer 23 may also be formed.

Each of the above semiconductor layers may be formed using ametalorganic chemical vapor deposition (MOCVD) technique, a hydridevapor phase epitaxy (HVPE) technique, a molecular beam epitaxy (MBE)technique, and the like.

The second conductive type semiconductor layer 29 and the active layer27 are patterned such that one region of the first conductive typesemiconductor layer 25 can be exposed to the outside. Such a patterningprocess is performed using photolithography and etching processes. Then,the electrode 31 is formed on the second conductive type semiconductorlayer 29, and the electrode pads 35 and 33 are formed on the electrode31 and the first conductive type semiconductor layer 25, respectively.The electrode 31 and the electrode pads 33 and 35 may be formed using alift-off technique.

As described above, the electrode 31 may be formed after the region ofthe first conductive type semiconductor layer 25 has been exposed.However, the electrode may be formed by forming an electrode layer usingan e-beam evaporation technique and then patterning the formed electrodelayer using photolithography and etching processes before the firstconductive type semiconductor layer 25 is exposed.

FIG. 7 is a sectional view illustrating a light emitting diode employinga patterned substrate 10 according to another embodiment of the presentinvention.

Referring to FIG. 7, a plurality of light emitting cells 28 are formedon the patterned substrate 10. The substrate 10 is the same as thesubstrate described referring to FIGS. 1 and 2. The substrate also hasthe protrusion patterns 17 and the recessed regions R1 and R2.

As described in FIG. 3, each of the plurality of light emitting cells 28comprises a first conductive type semiconductor layer 25, a secondconductive type semiconductor layer 29 formed on a region of the firstconductive type semiconductor layer 25, and an active layer 27interposed between the first and second conductive type semiconductorlayers. Here, the first conductive type semiconductor layers 25 of theplurality of light emitting cells 28 are spaced apart from one another.The plurality of light emitting cells 28 are electrically connected toone another through wirings 41 to form arrays connected in series. Thewirings 41 may be formed using an air-bridge or step-cover process.

Accordingly, there is provided a light emitting diode which can beconnected directly to and driven by an AC power source. An example of alight emitting diode that can be connected directly to and driven by anAC power source has been proposed in International Publication No. WO2004/023568 A1 entitled “Light-emitting device having light-emittingelements” by Sakai, et al. Descriptions on the operation of the lightemitting diode will be omitted herein.

1. A patterned substrate for a light emitting diode, comprising: a topsurface; a bottom surface; protrusion patterns arranged on the topsurface; and recessed regions surrounding the protrusion patterns andhaving irregular bottoms.
 2. The substrate as claimed in claim 1,wherein the substrate is a sapphire substrate.
 3. The substrate asclaimed in claim 1, wherein the bottom surface is a flat surface.
 4. Thesubstrate as claimed in claim 1, wherein the recessed regionssurrounding each of the protrusion patterns are symmetrical with respectto the relevant protrusion pattern.
 5. The substrate as claimed in claim1, wherein sidewalls of the recessed regions are formed into inclinedsurfaces.
 6. A light emitting diode, comprising: a patterned substratehaving a top surface, a bottom surface, protrusion patterns arranged onthe top surface, and recessed regions surrounding the protrusionpatterns and having irregular bottoms; a first conductive typesemiconductor layer formed on the top surface of the substrate; a secondconductive type semiconductor layer formed on a region of the firstconductive type semiconductor layer; and an active layer interposedbetween the first and second conductive type semi-conductor layers. 7.The light emitting diode as claimed in claim 6, further comprising abuffer layer interposed between the substrate and the first conductivetype semiconductor layer.
 8. A method of manufacturing a substrate for alight emitting diode, comprising: preparing a substrate having flat topand bottom surfaces; forming a first mask pattern on the top surface todefine first recessed regions; partially etching the substrate using thefirst mask pattern as an etching mask to form the first recessedregions; removing the first mask pattern; forming a second mask patternon the top surface to define second recessed regions partiallyoverlapping the first recessed regions; and partially etching thesubstrate using the second mask pattern as an etching mask to form thesecond recessed regions.
 9. The method as claimed in claim 8, whereinthe first mask pattern includes lines parallel to one another, and thesecond mask pattern includes lines which intersect the parallel lines ofthe first mask pattern.
 10. The method as claimed in claim 8, whereinsidewalls of the first and second recessed regions are formed intoinclined surfaces.
 11. A method of fabricating a light emitting diode,comprising: preparing a substrate having flat top and bottom surfaces;forming a first mask pattern on the top surface to define first recessedregions; partially etching the substrate using the first mask pattern asan etching mask to form the first recessed regions; removing the firstmask pattern; forming a second mask pattern on the top surface to definesecond recessed regions partially overlapping the first recessedregions; partially etching the substrate using the second mask patternas an etching mask to form the second recessed regions; forming a firstconductive type semiconductor layer, an active layer and a secondconductive type semiconductor layer on the substrate with the secondrecessed regions formed thereon; and patterning the second conductivetype semiconductor layer and the active layer to expose a region of thefirst conductive type semiconductor layer.